Synthesis and photophysical studies of covalently linked porphyrin-21-thiaporphyrin dyads

The synthesis and photophysical properties of four covalently linked unsymmetrical porphyrin dyads containing two different porphyrin cores such as N4 and N3S are reported. The covalently linked dyads were prepared by the coupling of appropriate porphyrin having ethynylphenyl functional group at meso-position with porphyrin having iodophenyl or bromo functional group at meso-position under mild palladium coupling conditions. The photophysical study indicated an intramolecular singlet–singlet energy transfer from N4/ZnN4 porphyrin sub-unit to N3S porphyrin sub-unit in all four dyads with an efficiency of energy transfer process was typically ⩾97%. To probe the role of linker in through bond electronic communication between the two porphyrin sub-units in dyads, the linker was varied from diphenylethyne to phenylethyne and the study revealed that the energy transfer rates and efficiencies were much higher for phenylethyne-bridged porphyrin dyads compared with diphenylethyne-bridged porphyrin dyads.